Monsoon constitutes an essential phenomenon for a tropical
climate (see Chapter 7, Section 7.6.3).
The monsoon precipitation simulated by AGCMs has been evaluated in AMIP (Sperber
and Palmer, 1996; Zhang et al., 1997; Gadgil and Sajani, 1998). The seasonal
migration of the major rain belt over the West African region is well simulated
by almost all models. However coarse resolution climate models generally fail
to give satisfactory simulations of the East Asian, East African and North American
monsoons (Stensrud et al., 1995; Lau and Yang, 1996; Semazzi and Sun, 1997;
Yu et al., 2000b). For example, models have excessive precipitation in the eastern
periphery of the Tibetan Plateau. Increase of horizontal resolution can improve
the precipitation details, but may not be sufficient to remove large-scale model
biases (Kar et al., 1996; Lal et al., 1997; Stephenson et al., 1998; Chandrasekar
et al., 1999; Martin, 1999; also see Section 8.9.1).

Interannual variations of Nordeste (north-eastern Brazil) rainfall are well
captured with atmospheric models with prescribed interannually varying SST (Potts
et al., 1996; Sperber and Palmer, 1996). This is also the case for the South
American monsoon (Robertson et al., 1999) and the West African monsoon (Rowell
et al., 1995; Semazzi et al., 1996; Rocha and Simmonds, 1997; Goddard and Graham,
1999). The precipitation variation over India is less well simulated. However
the models show better skill in reproducing the interannual variability of a
wind shear index over the Indian summer monsoon region, indicating that the
models exhibit greater fidelity in capturing the large-scale dynamic fluctuations
than the regional scale rainfall variations.

More recent atmospheric models with revised physical parametrizations show
improved interannual variability of the all-India rainfall, Indian/Asian monsoon
wind shear, Sahel and Nordeste rainfall (Figure 8.23)
(Sperber et al., 1999). Improvement in the simulation of interannual variability
is associated with a better simulation of the observed climate by the models
(Sperber and Palmer, 1996; Ferranti et al., 1999; Martin and Soman, 2000). The
observed rainfall/ENSO SST correlation pattern is better simulated by those
models that have a rainfall climatology in closer agreement with observations
(Gadgil and Sajani, 1998).

8.7.4 Madden and Julian Oscillation (MJO)

MJO is a 30 to 60 day oscillation that moves eastward in the tropical large-scale
circulation, and affects both mid-latitude atmospheric circulation and the Asian-Australian
monsoon. Slingo et al. (1996) showed that nearly all of the AMIP models have
power in the intra-seasonal time-scale of equatorial upper troposphere zonal
wind at higher frequencies than the observation. They also show that most models
underestimated the strength of the MJO. Slingo et al. (1999) show that the HadAM3
model forced by the observed SST displays a decadal time-scale variability of
MJO activity as observed, implying a possible link between long-term changes
of tropical SST and MJO activity, and also the ability of a current atmospheric
model to simulate it.

Recent studies suggest an important role of air-sea interaction on the intra-seasonal
time-scale phenomena (Flatau et al., 1997; Waliser et al., 1999; Li and Yu,
2001), thus a possible improvement in reproducing the MJO by coupled climate
models. This warrants a need to evaluate the MJO in coupled climate models,
but this is yet to be undertaken.

8.7.5 The North Atlantic Oscillation (NAO) and the Arctic
Oscillation (AO)

The North Atlantic Oscillation (NAO) is a regional mode of variability over
the North Atlantic, while the Arctic Oscillation (AO) is a hemispheric mode
of variability which resembles in many respects the NAO (see Chapter
7, Section 7.6.4). Coupled climate models simulate
the NAO quite well, although there are some differences in its amplitude (Delworth,
1996; Laurent et al., 1998; Saravanan, 1998; Osborn et al., 1999). Atmospheric
models with prescribed SST also simulate the spatial pattern of NAO variability
fairly well (Rodwell et al., 1999), although coupling to an interactive ocean
does seem to produce the most realistic NAO pattern. A realistic AO is simulated
in the CCCma (Fyfe et al., 1999), GISS (Shindell et al., 1999) and GFDL (Broccoli
et al., 1998) climate models. The AO extends into the mid-troposphere to lower
stratosphere where it is associated with variations in westerly wind speed (see
Chapter 2, Section 2.6.5 and Chapter
7, Section 7.6.4). This coupled troposphere-stratosphere
mode of internal variability has been reproduced in the Meteorological Research
Institute (MRI) coupled climate model (Kitoh et al., 1996; Kodera et al., 1996).